Growth and characterization of GaN single crystals

Up to 3 mm long GaN single crystals were grown by sublimation of cold pressed GaN pellets or evaporation of gallium (Ga) metal under an ammonia (NH₃) flow in a dual heater, high-temperature growth system. A growth rate of 500 μm/h along the [0 0 0 1] direction was achieved using a source temperature of 1200 °C, a total pressure of 760 Torr, and an NH₃ flow rate of 50 sccm. The resulting crystals were transparent, possessed low aspect ratios and well-defined growth facets. The only impurity present at high concentrations was oxygen (3×10¹⁸ atoms/cm³). Photoluminescence studies conducted at 77 K showed a sharp emission peak centered at 359 nm. Time-dependent photoluminescence measurements revealed optical metastability in bulk GaN. Raman spectroscopy yielded narrow peaks representing only the modes allowed for the wurtzite structure. All characterization studies confirmed excellent crystalline and optical quality of the obtained single crystals.